1. Field of the Invention
The present invention relates to a power transmission shaft in a steering unit that shortens in an axial direction when impacted by a specified impact load and thereby absorbs impact, and a method for assembling thereof. The power transmission shaft herein is an intermediate shaft that is arranged between a steering shaft and a steering gear in an automobile steering unit. A steering column is arranged between the steering shaft and a steering wheel.
2. Description of Related Art
FIG. 4 shows a structure of a conventional steering unit in an automobile. In the FIG., 1 represents a steering wheel, 2 a steering column, 3 a steering shaft, 4 a steering gear, 5 and 6 universal joints respectively, and 7 represents an intermediate shaft.
The intermediate shaft 7 transmits turning power from the steering wheel 1 to the steering gear 4. Upon excessive impact owing to a collision, the intermediate shaft 7 shortens to thereby absorb impact so that such impact is not transmitted to the automobile driver.
FIG. 5 is a side view of an important portion of the intermediate shaft 7. The intermediate shaft 7 comprises a hollow shaft 8 and an insertion shaft 9 which are so connected as to be able to move in an axial direction respectively. A female serration is arranged on the inner circumference of the hollow shaft 8, while arranged on the outer circumference at the end of the insertion shaft 9 is a male serration 9a that engages with the female serration of the hollow shaft 8. A circumferential slot 10 is formed on the outer circumference of the insertion shaft 9 in the area having the male serration 9a. In the hollow shaft 8, two radial holes 11 are arranged at two positions that are opposite by 180 degrees. The holes 11 are positioned to correspond to the above-mentioned circumferential groove 10 when the insertion shaft 9 is inserted into the hollow shaft 8. Through the holes 11, a resin 12 is filled between the circumferential groove 10 and the hollow shaft 8. When the resin 12 is hardened, the hollow shaft 8 and the insertion shaft 9 are connected integrally.
In the intermediate shaft 7 of the structure mentioned above, when excessive impact occurs, the resin 12 is sheared, and the insertion shaft 9 goes into the hollow shaft 8. By this occurrence, the entire intermediate shaft 7 shortens, and thus absorbs the impact.
In the above conventional example, the resin 12 must be hardened in the course of production. Accordingly, the conventional example suffers bad working efficiency, and causes high production costs.
Further, the conventional steering unit, assembled in an engine room, will be subjected to high temperatures. Therefore, it is necessary to prevent deterioration of the strength of the resin 12. When the strength of the resin 12 is deteriorated for some reason, a desired, specified shear resistance may not be attained, leading to unpredictable variations of draft load. Draft load means an impact load sufficient to shear the resin 12 and thereby shorten the intermediate shaft 7. Moreover, when the resin 12 is sheared, draft load is apt to decline sharply, therefore, sufficient considerations must be paid to impact absorption.
The steering column 2 shown in FIG. 4 comprises a hollow shaft and an insertion shaft similar to the above-mentioned intermediate shaft 7. The hollow shaft and insertion shaft are connected integrally by use of the resin 12 as shown in FIG. 5 to form an impact absorbing structure. As a consequence, the steering column 2 also has nonconformities similar to those mentioned above.